1. Field of the Invention
The present invention relates to a projection display apparatus for projecting an optical image formed on a light valve onto a screen in a magnified state by a projection lens.
2. Description of the Related Art
Conventionally, various methods for projecting an optical image formed on a light valve onto a large screen using a projection lens to display a video image in a magnified state are known. As a display apparatus used for such methods, a projection display apparatus including a liquid crystal panel as a light valve is known today (disclosed in, for example, Japanese Laid-Open Patent Publication No. 60-2916 and Japanese Patent Publication No. 4-35048).
With reference to FIG. 30, a basic structure of such a projection display apparatus will be described.
As is illustrated in FIG. 30, the projection display apparatus includes a light source 502, a liquid crystal panel 503, and a projection lens 504. A field lens 505 is used for effectively transmitting light which has passed through the liquid crystal panel 503 to the projection lens 504.
The light source 502 includes, for example, a lamp 506 and a concave mirror 507, and outputs light for illuminating the liquid crystal panel 503. Recently, use of a metal halide lamp as the lamp 505 is being expanded due to the high luminous efficacy and satisfactory color reproduction properties thereof. Instead of the metal halide lamp, a halogen lamp or a xenon lamp, for example, may be used. A UV (ultraviolet)-IR (infrared) cutting filter 508 is used for removing infrared rays and ultraviolet rays from the light emitted by the light source 502.
The liquid crystal panel 503 forms an optical image by changing the light transmittance spatially in accordance with a video signal and projects the optical image on a large screen by the projection lens 504 to display a video image in a magnified state.
As the liquid crystal panel 503, a twisted nematic (hereinafter, referred to as "TN") liquid crystal panel driven by an active matrix system is mainly used. Such a type of liquid crystal panel has an advantage of providing a relatively high contrast between a white area and a black area and thus displaying a high quality video image.
Such a conventional projection display apparatus has a problem of a low brightness of the image formed on the screen. Especially, a TN liquid crystal panel displays an image using light polarization. For this reason, approximately 50% of the natural light which is incident on the liquid crystal panel is lost. Generally, as the density of pixels increases, the aperture ratio of each pixel (the ratio of the area of an aperture of the pixel with respect to the total area of the pixel) decreases. A part of the light which is not transmitted through the aperture is reflected or absorbed by a light shielding layer of the liquid crystal panel and thus is lost. The light transmittance of a general TN type liquid crystal panel is less than 10%. In other words, a major part of the light emitted by the light source is lost.
Accordingly, in order to produce a projection display apparatus having a large luminous output, a light source which provides a large luminous output for illuminating the light valve is needed. A light source having a large luminous output requires a lamp having a high luminous efficacy and a high level of power, and efficient condensing of the light emitted by the lamp.
In general, a lamp having a high level of power has a relatively great total length, which prevents formation of a compact projection display apparatus. Since a lamp having a higher level of power is usable in fewer applications, it is difficult to obtain a lamp which fulfills the desired specifications and performance. These problems can be solved by newly developing a lamp having a desired level of power, but such development requires a relatively long time period. Further, such a lamp having special specifications is more costly than multi-purpose lamps.
A metal halide lamp has a luminous efficacy which is approximately three times the luminous efficacy of a xenon lamp or a halogen lamp. Accordingly, the amount of light emitted by a light source including a metal halide lamp is approximately three times higher if the level of power of the lamp is the same. However, the above-described problems are more serious when a metal halide lamp is used.
The light emitting characteristics of the metal halide lamp depend heavily on factors such as the type or the amount of the metal added, the shape of the tube, the arc length, and the level of power of the lamp. While a multi-purpose lamp having a power of 250 W which fulfills the desired light emitting characteristics is available relatively easily, it requires a long time and investment of a large sum of money to newly develop a lamp having a power of 500 W or 1 kW and also the equivalent light emitting characteristics. Accordingly, the cost of the lamp and a circuit for turning the lamp on increases, and thus the cost of the projection display apparatus is raised.
Generally, as the level of power of the lamp rises, the size of a luminous element in the lamp increases. In the case of a metal halide lamp, the length of the luminous element increases substantially in proportion to the level of power of the lamp. If the level of power of the lamp is raised without increasing the length of the luminous element, the light emitting characteristics are deteriorated and thus the life of the lamp is shortened.
When the luminous element is large, light emitted by various parts of the luminous element proceeds in various directions. Thus, it is difficult to condense the light efficiently. For example, in the case that light emitted by a lengthy luminous element of a metal halide lamp is condensed by a parabolic mirror, the light reflected on the parabolic mirror is relatively highly parallel in the direction corresponding to the direction of a shorter axis of the luminous element, but is not parallel sufficiently in the direction corresponding to a longer axis of the luminous element. In such a case, in order to raise the light utilization efficiency, a further optical system is needed downstream with respect to the parabolic mirror in a direction in which the light is transmitted to utilize a light beam running at a largest angle with respect to an axis of the light. For example, the projection lamp needs to have a larger diameter so as to correspond to the largest angle. A lens having such a large diameter is not preferable because the restriction of various aberrations is difficult and such a lens is generally costly.
For the above-described reasons, a luminous element preferably has a minimum possible size. The light emitting surface thereof preferably has a shape which is as close as possible to a globe because the expanding angle of light emitted by a globe-shaped luminous element is averaged in all the directions. A metal halide lamp having a high level of power is not preferable because a lengthy luminous element thereof prevents efficient light condensing.
As an apparatus having a high efficiency of light condensing and realizing light radiation which provides the light receiving surface with a uniform brightness, an optical integrator including two lens arrays is known (disclosed in, for example, in the U.S. Pat. No. 5,098,184). In an optical integrator, a light beam obtained by condensing light which is emitted by the lamp is divided by two lens arrays to average the brightness and is then transmitted to the light receiving surface.
Such an optical integrator has the above-described problems when bright light radiation using a lamp having a high level of power is needed. Since the total length of the lamp is large, it is difficult to form a compact light source. A larger light source requires an optical element such as a lens array having a larger effective diameter, which raises the cost. In order to utilize light emitted by a lengthy luminous element efficiently, the effective aperture of the lens array needs to be enlarged. This causes problems in ease of production and production cost of the lens array.
Especially in the case of a metal halide lamp, luminous output variation is difficult. If a different level of power from the prescribed level of power of the lamp is provided, luminous output changes but problems occur such that the emission spectrum changes significantly, the light cannot be kept on, the life is shortened significantly, and the lamp is easily broken. Accordingly, a projection display apparatus including such a lamp has a problem that adjustment of brightness of an image formed on the screen is difficult. Although the brightness is lowered by decreasing the light transmittance of the light valve, the display quality of the image formed on the screen by projection is also lowered. If a separate element for adjusting the brightness, for example, a variable diaphragm is added, the number of components and the cost increase.